Effect of Ta on microstructure and phase distribution in cyclic oxidized Ir-Ta modified aluminide coatings on nickel base single crystal superalloy

The effect of Ir-Ta coating (with varying amounts of Ta) on microstructure, composition variation and phase distribution in the modified aluminides on Ni-base single crystal superalloy, TMS-75 subjected to cyclic oxidation has been investigated. Ir-Ta coatings with Ta = 14.1, 23.1, 40.7 and 82.9 at.% deposited on 〈1 0 0〉 oriented nickel base single crystal superalloy, TMS-75 were aluminized and subjected to cyclic oxidation at 1373 K, for 600 h. The mass gain as a function of number of cycles data demonstrates an improved oxidation resistance with decreasing Ta content. In contrary to Ta-rich aluminides, Ir-rich aluminides show a large volume fraction of the β-phase, a less amount of γ′-Ni₃Al phase near the surface of the coatings. Rapid decrease in the concentration of Al in the diffusion zone and formation of interconnected γ′-Ni₃Al has been found to be responsible for the lower oxidation resistance of Ta-rich aluminides.